In common dry cells, external cases are composed of thin steel sheets surface-treated with nickel plating, etc. The external cases can be produced by deep-drawing the thin steel sheet materials into cylindrical shapes, ironing being further carried out optionally thereafter. In the production process such as the deep drawing, the thin steel sheers come into contact with press dies and tools, and are subjected to strong forces, to be greatly deformed.
Incidentally the thin steel sheet materials to be subjected to the deep drawing often have defects such as fine pores of blow holes and non-metal inclusion contaminations, which are inevitably generated in the production of the materials. When the thin steel sheet material has such defects, scratches such as cracks are often formed around the defects during the process of deep drawing, etc. The scratches are so fine that it is almost impossible to visually find the scratches, even surface ones. Further, even when the thin steel sheet material has no defects, the material comes into contact with press dies, etc. during the forming process, whereby in some cases scratches are formed in the vicinity of the surface of the cylindrical-shaped material.
In a case where a dry cell is used for a long period, a liquid might leak from the cell due to deterioration of an electrolyte, etc., and thereby an electronics device using the dry cell as a power source is contaminated and deteriorated. Further, in a case where a plurality of dry cells are used in series and one cell is connected in the incorrect opposite polar direction, a reverse current passes through the cell, and thereby a gas can be generated to rupture the cell. The thin steel sheets used for the external cases of the dry cells have small thickness of 0.1 to 0.2 mm, whereby the scratches on the external cases can cause liquid leakage and rupture. Thus, the dry cell external cases having scratches in the vicinity of the surfaces must be rejected as inferior products.
Magnetism search methods have been known as methods for inspecting scratches on steel sheet surfaces. As shown in FIG. 6, a magnetic flux in the vicinity of a steel sheet surface is distorted and leaks from the surface in a position with a defect such as a crack, and the above methods utilize this phenomenon. By detecting the leak magnetic flux with a magnetic sensor, the scratches in the vicinity of the steel sheet surface can be inspected. The leak magnetic flux may be detected by spreading iron powder and by visually observing its aggregation state, instead of using the magnetic sensor.
A device for detecting a leak magnetic flux by using a magnetic sensor, thereby inspecting a scratch on a steel sheet surface, is disclosed in JP-A-56-1645. The device, shown in FIG. 7, is used for inspecting a scratch on an object material of a steel sheet 20. A detection unit 21 is attached to a guide bar 8 and placed above the steel sheet 20. The detection unit 21 has a C-shaped frame 4, which has ends of legs immediately above the steel sheet 20, and an exciting coil 5 is attached to the center of the frame 4. Further, the detection unit 21 has a plurality of magnetic sensors 6 arranged in the vicinity of the steel sheet 20, aid the distance between each magnetic sensor 6 and the steel sheet 20 is 1 mm or less.
To inspect a scratch, the frame 4 is converted to an electromagnet by applying a current to the exciting coil 15. Thus, in the case of using the leg 4A as an N pole, a magnetic flux passes from the leg 4A into the S pole leg 4B, across the surface of the steel sheet 20. In this case, when the steel sheet 20 has a scratch (or a defect), the magnetic flux leaks from the surface of the steel sheet 20, and the leak magnetic flux is detected by the magnetic sensors 6. The detection signal is processed by a signal processing device (not shown), to measure position of a scratch. The detection unit 21 is guided by the guide bar 8 and moved in the width direction of the steel sheet 20, and scans the steel sheet 20 to inspect a scratch.
Patent Document 1: JP-A-56-61645